Why is the British government still using IE?

A recent article in the Guardian revealed that the British government and armed forces are continuing use of IE 6, despite known security holes.

Of course, the solution here is not to upgrade to later versions of IE. The solution is to switch to a free browser like Icecat or Firefox -- then the government would have full access to the source code, which translates to quicker fixes to any security problems, and earlier warnings about them. Microsoft's definition of security is about Microsoft, not its users.

(The Guardian piece unfortunately uses the word "hacker" to mean "attacker" rather than "someone who enjoys playful cleverness.")